Abstract: A forced air fireplace heater is disclosed which can be easily installed into a variety of sizes of existing fireplaces. A substantially vertical plenum approximately as wide as the firebox is installed in the rear portion thereof. A forced air blower is connected to the lower portion of the plenum. A plurality of heat exchanger tubes are connected to outlet ports on the top of the plenum and extend forwardly from the plenum across the throat of the fireplace so as to direct forced air introduced by the blower and heated in the plenum and heat exchanger tubes, past the lintel of the fireplace and into the ambient. The heat exchanger tubes are pivotally mounted to the plenum and have an adjustable length so as to permit the assembly to be installed in a variety of fireplace sizes and to be adjusted so as to optimally extract waste heat from the combustion gases.